NL8320288A - PROCESS FOR MANUFACTURING A PREFERABLY-HEAT-INSULATING TUBE. - Google Patents

Description

8 3? ο. : .. C; C, V.O. 6328

Title: Method for manufacturing a preferably flexible heat insulating tube.

The present invention relates to a flexible heat-insulating pipe, in particular for underground district heating pipe systems, in which it can be advantageous to use such flexible pipes for small connecting pipes to facilitate the construction.

Known flexible tubes for this purpose consist of a thin-walled inner tube, the flow tube, of steel or copper, an envelope insulation in the form of polyurethane foam and a protective outer jacket tube of a suitable plastic, usually a corrugated polyethylene tube. It is obvious to use soft polyurethane foam as an insulating material for flexible pipes, while hard foam is used on the other hand in district heating pipes, and it is necessary to accept the drawbacks that the specific weight of the soft foam is considerably higher than that of rigid foam and last but not least soft foam is clearly hygroscopic. Some manufacturers therefore prefer rigid polyurethane foam, and another objection must be accepted, namely that the foam will break if the tube is bent, which is disadvantageous for several reasons.

The object of the invention is to provide a flexible tube which does not have these disadvantages.

The invention is based on the recognition of the fact that special foam materials have already been developed, which in the present context are particularly suitable as insulating material, i.e. soft and light closed-cell materials, which are sufficiently heat resistant.

In particular, a material known under the trademark FRELEN is mentioned, which is a foam of so-called "cross-linked" polyethylene and which is produced in the form of plates. The plates are semi-soft and resilient, and they can be tubularly rolled, the opposite edge surfaces of the plate being joined together in a radial plane by gluing or otherwise.

Such foam tubes will be useful as insulation on or in a flexible tube and an improvement will already be achieved as a result, both with respect to soft and hard polyurethane foam. However, the invention 8320288-2- goes a step further by providing a method of manufacturing the insulated tube, thereby providing an additional advantageous product.

According to the invention, this method is characterized in that the insulating foam tube is tightly placed around the iron or copper tube, after which the tube is heated until the inside of the foam tube starts to melt, such that the foam tube from the inside melts to the outside of the inner tube. Such a method of joining has proved to be accessible at least when the above-mentioned cross-linked polyethylene is used.

By this method, the important advantage is obtained that the inner layer of the non-hygroscopic foam material is applied to the inner pipe in the form of a rigid covering layer, so that in practice it will be unimportant if a small leak occurs in the inner pipe, for example a crack created by bending the thin-walled tube. Such a leak will be pre-sealed by the inner foam layer, and if the liquid pressure in the inner pipe is reasonably low, the liquid will consequently be unable to flow out into the foam due to the closure of the cells of this foam, or along the outside of the inner pipe, because of the adhesion of the foam material to the pipe surface in the area around the leak. With pipes insulated with polyurethane foam, the outflow of liquid can hardly be prevented, and in particular not when using soft foam; when using hard foam, a leakage will progress more slowly, but will not be prevented, ie the insulating capacity of the foam will decrease in an area of increasing size and a local reduction in insulating capacity will occur quickly on the flexible tube due to of the cracks formed in the foam when bending the tube. When applying the invention the associated necessary and time consuming repair work can be completely avoided. An additional advantage of the invention is that it suffices to use a relatively inexpensive inner tube, for example a tube with reduced wall thickness and even a tube with increased flexibility.

A further aspect of the invention is that it is possible in principle to produce a tight connection between the outside of the foam tube and the inside of the outer jacket tube in a desired manner. The casing is usually available as an extruded polyethylene tube, and especially if the foam tube 5 consists of polyethylene, such a tight connection can be obtained by extrusion of the casing tube directly along the foam tube, such that the foam tube automatically warms up to the the inside of the casing is welded. This welding is achieved even if the jacket pipe is treated with special tools immediately after leaving an extrusion nozzle placed every 10 foam tube to obtain a waveform.

In the following, the invention is more particularly described with reference to the drawing, in which: Figures 1 and 2 illustrate different stages of the manufacture of a tube according to the invention; Fig. 3 is a longitudinal section through a part of a manufactured pipe section; Fig. 4 is a side view illustrating a further method step; FIG. 5 is a longitudinal section showing the manufacture of an outer jacket associated with the tube and FIG. 6 is a perspective view showing a modified method of manufacturing the insulated tube.

Fig. 1 shows that an elongated flexible metal tube 2 on one side, the right side, is connected to a source 4 of compressed air, while the tube on its other end is enveloped by the end portion 6 of a foam tube 8, the opposite of which end at 10 is tightly closed, while the end portion 6 clamps around the outside of the metal tube 2. The natural inner diameter of the foam tube 8 is slightly smaller than the outer diameter of the metal tube 2, but due to the air pressure from the source 4, the resilient foam tube 8 will expand slightly so that it easily over and along the metal tube 2 can be drawn, while the foam tube is kept inflated behind the end portion 6 clamping around the tube 2.

8320288 -4-

As noted above, the foam tube 8 may be formed by rolling up plate-shaped foam elements of limited length and thickness.

The pipe may be longitudinally composed of various rolled sheet elements, as indicated by joining surfaces 5 or adhesive joints 12. Should the wall thickness of the foam pipe 8 be insufficient to provide the required insulation of the pipe 2, an additional foam pipe 14 with connections 16 are applied to the outside of the foam tube 8 by a mounting method analogous to that of FIG. 2.

As can be seen from Fig. 3, care can be taken that the connections 12 and 16 of the two foam tubes 8 and 14, respectively, eventually become offset in an axial sense.

If the tube 2, which can for instance be 50 m long, is thus provided with one or more insulating tubes 8, 14, the two ends of the metal tube 2 are subjected to the electrical voltage of a current generator 18, such that the tube 2 thereby is heated until the inner surface layer of the foam tube 4, which already includes the outside of the metal tube 2 resiliently, begins to melt. As a result, the foam material will become connected to the outside of the metal tube 2.

If the foam tube or tubes 8, 14 consist of the said polypropylene material, the foam will be soft and resilient so that it is easily and easily tearable together with the inner tube 2 and it will comprise closed cells that move liquid will prevent through the material. Moreover, if the foam material is bonded to the surface of the inner tube 2, a possible leak in the tube wall will not cause a permanent outflow of liquid into the foam or along the surface of the tube 2, at least if the pressure in the pipe is so low as usual with district heating systems.

The heat-insulated pipe described so far can be provided with a protective outer wall, for example by simply placing it in a separately manufactured plastic jacket pipe, which is preferably formed as a corrugated pipe to facilitate the bending of the complete pipe.

8320288 -5-

Since the outer jacket tube is conventionally and suitably made of polyethylene and the preferred foam material is the same material, the invention provides a precondition for the favorable ability to provide a desired close connection between the desired mutually adjoining surfaces of the foam tube and the casing tube, for example by gluing. A special possibility is to achieve thermal fusion of these surfaces, for example by passing the extruded jacket pipe 20, as shown in Fig. 5, through a ring nozzle 10 and directly around the foam tube 8, 14 when they are moved centrally through this nozzle, whereby the hot outer jacket material fuses with the outside of the foam tube. In a known manner, the casing tube can be machined by corrugated jaws 24, which transform the smooth tube into a more bendable corrugated tube.

For the correct operation of the jaws it may be advantageous if the soft foam tube has a somewhat harder surface layer and the outer foam tube element can therefore have a higher specific weight compared to the inner element or elements.

In FIG. 6, it is indicated that starting from a set of foam plates 26 in strip form and using mold or guide tooling 28.mogéLgk is a gradual formation of a: foam tube 30 to produce the inner tube 2 as the latter is advanced between the tools 28, the sheet-25 plate strips being successively formed into two half-shells and then welded together by passing a fixed set of welding tools 32. Subsequent heating of the inner tube can be accomplished as shown in FIG. by electric current or by choice, by means of hot oil. Even high-frequency heating in a 30 field. next to or in front of the welding tools 32 is possible.

The use of the said special insulating material will also be advantageous if the inner tube consists of plastic, and of course the adhesion of the outer jacket tube, fig. 5 will depend on the character of the inner tube.

With known heat-insulating pipes, the outer jacket is necessary both to prevent the ingress of water into the 8320288 if-6 insulating material and to protect the latter in a mechanical sense. When using the invention, consideration may be given to omitting the outer jacket if mechanical protection of the foam tube is unnecessary. Conversely, it should be noted that the said possibility of adhering the outer plastic tube to the foam insulation by fusion welding in connection with extruding the outer tube has the special advantage that a perforation of the outer tube would not give rise to the penetration of water from outside in the outer tube; although water penetrating in this way could not penetrate into the insulating material itself because of the closed cells, it could however still propagate along the inner side of the outer tube and thus achieve a connected pipe system of a different and more water-sensitive nature. It will be understood that the described solution to this problem is equivalent to avoiding the propagation of water along the inner tube according to the first described aspect of the invention.

8320288

Claims (5)

A method for manufacturing a preferably flexible heat insulating tube in which a metal inner tube, the flow tube, is covered with a tube of insulating foam material of cross-linked polyethylene or a comparable thermoplastic foam with closed cells, which is water and heat resistant at normal operating temperature, characterized in that the inner tube during or after the application of the foam tube is overheated to such an extent that the inner surface of the foam cover melts on the outside of the inner tube. 2. Method according to claim 1, characterized in that the heating of the inner tube is effected by passing a heating medium or an electric current through the pipe after it has first been provided with the foam insulation over the entire length, preferably a large production length of, for example, 50 m. Method according to claim 1 or 2, characterized in that the foam material cover is drawn over the flow tube in the form of a preformed, elastically expandable hose, the inner diameter of which, when relaxed, is smaller than the outer diameter of the flow tube . 4. Method according to claim 3, characterized in that compressed air is admitted into the space between the respective ends of the foam hose and the flow pipe during the drawing-up of the foam hose, wherein these ends are temporarily closed. 5. A method according to claim 1, characterized in that the thermoplastic foam material is additionally heat-welded with a plastic outer jacket tube, which is formed directly on the outside of the foam tube by hot extrusion. 83 2 0 2 8 8